A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the ossicles of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window membranes of the cochlea 104. The cochlea 104 is a long narrow organ wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, various types of hearing prostheses have been developed. For example, when hearing impairment is associated with the cochlea 104, a cochlear implant with an implanted stimulation electrode can electrically stimulate auditory nerve tissue within the cochlea 104 with small currents delivered by multiple electrode contacts distributed along the electrode.
When a hearing impairment is related to the operation of the middle ear 103, a conventional hearing aid or a middle ear implant (MEI) device may be used to provide acoustic-mechanical vibration to the auditory system. FIG. 1 also shows some components in a typical MEI arrangement where an external audio processor 100 processes ambient sounds to produce an implant communications signal that is transmitted through the skin to an implanted receiver 102. Receiver 102 includes a receiver coil that transcutaneously receives signals the implant communications signal which is then demodulated into a transducer stimulation signals which is sent over leads 106 through a surgically created channel in the temporal bone to a floating mass transducer (FMT) 104 in the middle ear. The transducer stimulation signals cause drive coils within the FMT 104 to generate varying magnetic fields which in turn vibrate a magnetic mass suspending within the FMT 104. The vibration of the inertial mass of the magnet within the FMT 104 creates vibration of the housing of the FMT 104 relative to the magnet. And since the FMT 104 is connected to the incus, it then vibrates in response to the vibration of the FMT 104 which is perceived by the user as sound.
Besides the inertial mass magnet within an FMT, some hearing implants such as Middle Ear Implants (MEI's) and Cochlear Implants (CI's) also employ attachment magnets in the implantable part and an external part to hold the external part magnetically in place over the implant. For example, as shown in FIG. 2, a typical MEI system may include an external transmitter housing 201 containing transmitting coils 202 and an external magnet 203. The external magnet 203 has a conventional disk-shape and a north-south magnetic dipole that is perpendicular to the skin of the patient to produce external magnetic field lines 204 as shown. Implanted under the patient's skin is a corresponding receiver assembly 205 having similar receiving coils 206 and an implanted internal magnet 207. The internal magnet 207 also has a disk-shape and a north-south magnetic dipole that is perpendicular to the skin of the patient to produce internal magnetic field lines 208 as shown. The internal receiver housing 205 is surgically implanted and fixed in place within the patient's body. The external transmitter housing 201 is placed in proper position over the skin covering the internal receiver assembly 205 and held in place by interaction between the internal magnetic field lines 208 and the external magnetic field lines 204. Rf signals from the transmitter coils 202 couple data and/or power to the receiving coil 206 which is in communication with the implanted MEI transducer (e.g., the FMT, not shown).
A problem arises when a patient with a hearing implant undergoes Magnetic Resonance Imaging (MRI) examination. Interactions occur between the implant magnet(s) and the applied external magnetic field for the MRI. As shown in FIG. 3, the direction magnetization m of the implant magnet 302 is essentially perpendicular to the skin of the patient. Thus, the external magnetic field B from the MRI may create a torque T on the internal magnet 302, which may displace the internal magnet 302 or the whole implant housing 301 out of proper position. Among other things, this may damage the adjacent tissue in the patient. In addition, the external magnetic field B from the MRI may reduce or remove the magnetization m of the implant magnet 302 so that it may no longer be strong enough to hold the external transmitter housing in proper position. The implant magnet 302 may also cause imaging artifacts in the MRI image, there may be induced voltages in the receiving coil, and hearing artifacts due to the interaction of the external magnetic field B of the MRI with the implanted device. This is especially an issue with MRI field strengths exceeding 1.5 Tesla.
Thus, for existing implant systems with magnet arrangements, it is common to either not permit MRI or at most limit use of MRI to lower field strengths. Other existing solutions include use of a surgically removable magnets, spherical implant magnets (e.g. U.S. Pat. No. 7,566,296), and various ring magnet designs (e.g., U.S. Provisional Patent 61/227,632, filed Jul. 22, 2009). Among those solutions that do not require surgery to remove the magnet, the spherical magnet design may be the most convenient and safest option for MRI removal even at very high field strengths. But the spherical magnet arrangement requires a relatively large magnet much larger than the thickness of the other components of the implant, thereby increasing the volume occupied by the implant. This in turn can create its own problems. For example, some systems, such as cochlear implants, are implanted between the skin and underlying bone. The “spherical bump” of the magnet housing therefore requires preparing a recess into the underlying bone. This is an additional step during implantation in such applications which can be very challenging or even impossible in case of very young children.